If you're not completely changing colors totally (ex. dark blue to yellow) you could have an active object set on a layer over your gameplay area that's say a solid black and change THAT object's transparency. Just make sure the active fills the visible screen area and set it to not follow the frame (like a HUD) so it always stays in one place over everything.

So during the daytime it'd be transparency value 128 (invisible) and when it gets darker you subtract from that. Obviously you wouldn't want it to hit 0 else you wouldn't be able to see anything at all but you get the idea.